world, and seeing them harkens memories of the late Permian world prior to the great Permian mass extinction. All are panting heavily and give the impression of animals having just engaged in strenuous exercise. But most have been motionless; yet they pant for good reason. The level of atmospheric oxygen at this time is equivalent to being higher than 10,000 feet in our world. Except that here we are not atop any mountain: the low swamps and nearby arm of the sea attest to our being at sea level.

Other herbivores are here too, and they are clearly from groups long-diverged away from the mammal-like reptile lineage. It is soon apparent that they are more numerous than the mammal-like reptiles. One of the oddest is another beaked herbivore, from the reptile group known as rhynchosaurs, and near it is a heavily armored quadruped, an aetosaur, looking something like an armadillo, only much larger and better armored. Soon we notice other quadruped reptiles, all fairly primitive archosaurs. Many are 5 to 10 feet in length, and they too move little; when they do, the movement is labored and the panting rapid. Large size and armor evolve for one reason, to avoid being eaten—but the cost is high. Moving a heavy body about extracts a great metabolic cost. Yet there is method in this seemingly morphological madness, for it is clear that the carnivores here are in abundance.

A number of reptiles are visible with heads like that of a crocodile but with bodies obviously evolved for rapid movement on land. Some rise up on their back legs, but they are still quadrupeds for rapid movement. They seem better suited for prolonged movement than the other designs seen till until now, but they are no greyhounds. They prowl but in labored fashion.

Until now all the animals we have seen have been quadrupeds, but it is not long before we encounter our first bipedal animal, and soon we find that this world certainly has its share of animals that walk bipedally. All of them seem far more at ease in this atmosphere, one that bothers even us humans, supposedly advanced mammals that we are. We have finally encountered our first dinosaurs.

Some are small and carnivorous; others, the prosauropods, are relative giants, the largest animals on land and the largest animals ever evolved up to this point. There seem to be many varieties of the smaller



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement